Found another post that answered my question just after posting this!

Data - Data Validation - Input Message

"tommcbrny" wrote:

I want to change the comments / instructions that pop up whenever I select a
cell on a spreadsheet. I figured out the setting once, but can't find it
again and it's killing me.

Can someone tell me what this function is called (it's not a comment on the
cell, but rather a text box with information that appears whenever a cell is
selected) and how to access the text to change it.

Thanks, and sorry for the dumb question.
Tom